1. Field of the Invention
The present invention relates to offshore drilling vessels, and more particularly to methods and apparatus relating to fairleads.
2. Description of the Related Art
An offshore drilling vessel is essentially a massive, floating, mobile vessel used in the offshore exploration and/or production of oil and gas. The vessel is equipped with the necessary drilling tools to drill an oil and gas well into the ocean floor or produce the oil and gas to the surface for recovery. Up until the past few years, the typical offshore drilling vessel was of the type that generally includes two large pontoon hulls, at least four vertical support columns, and a drilling platform. The pontoon hulls float in the water horizontally and are parallel to one another. At least two support columns are attached to and extend vertically upwardly from each pontoon hull. The drilling platform is horizontally attached upon the vertical columns. This type of vessel is sometimes referred to as a "semi-submersible" vessel, and is towed from drilling site to drilling site by one or more boats.
Within the past few years, another type of offshore drilling and production vessel has been introduced to the market; this type of vessel is sometimes referred to as a spar-type vessel. This type of vessel is similar to the older semi-submersible-type vessel in that it includes a drilling and/or production platform, but differs in the flotation mechanism upon which the platform rests. More particularly, instead of using pontoon hulls and four vertical supports to support the platform, the spar-type vessel supports the platform with a single, long, slender cylinder, or support column, that is vertically moored in the water. This support column is sometimes referred to as a Deep Draft Caisson Vessel, or DDCV. As just one example, the cylinder may have a diameter of approximately 120 feet, and a length of approximately 500 feet. In this example, when moored in the drilling and/or production position, there may be approximately 50 to 80 feet of the cylinder exposed above the water line, with the remainder disposed below the water line.
Irrespective of the type of vessel, whether it be the older "semi-submersible" type or the newer "spar" type, the vessel must be anchored to the ocean floor before drilling begins. With a semi-submersible vessel, which is generally rectangular in shape, there will be at least one large mooring line, and sometimes more than one mooring line, at each corner of the vessel. Each mooring line is in turn connected to a large anchor at the ocean floor. With the spar-type vessel, there will be a number of anchors, perhaps as many as twelve, attached about the circumference of the support column, or DDCV, upon which the platform rests. With both types of vessels, each mooring line is tensioned or relaxed by its own mooring unit, through the use of a wire rope, chain, or combination wire rope/chain mooring line. In very broad terms, a mooring unit is essentially a giant hoist. Each type of vessel is also equipped with a "fairlead" for each mooring unit. A fairlead is essentially a pulley or sheave. The fairleads are mounted to the vessel directly below each mooring unit. With the semi-submersible vessels, the fairleads are mounted to the vertical support members. With the spar-type vessels, the fairleads are mounted directly to the vertical support column, or DDCV. For each mooring unit, the mooring line exits the mooring unit and passes around its corresponding fairlead pulley or sheave before being connected to the anchor.
The present invention relates to fairleads, and the need for them to be accessible in the event they may experience mechanical difficulty and require repair. With the older semi-submersible type of vessel, the fairleads are generally located approximately ten to twenty feet below the water line when the vessel is in the moored position. In the event one of the fairleads needs to be repaired, the vessel can be raised, or "ballasted", out of the water far enough to elevate the fairleads above the water line; this "ballasted" position is the position in which semi-submersible vessels are placed when they are being moved from one drilling site to another. Once the vessel is ballasted, the fairlead may then either be repaired at the drilling site or the vessel could easily be moved to a dock for repair of the fairlead(s). With the newer spar-type vessel, however, the situation is different. When the spar-type vessel is in its moored or drilling position, the fairleads are located much farther below the water line; for the sample dimensions given above, the fairleads may be located approximately 120 feet below the water line. Further, it is not practical or feasible to "ballast" a spar-type vessel, whereas there is little difficulty in "ballasting" a semi-submersible vessel. In fact, it is not uncommon for spar-type vessels to be designed to be stationary in their moored positions for as long as twenty years. As such, the above-discussed approach to repairing fairleads on semi-submersible vessels is simply not workable for spar-type vessels. Accordingly, there has developed a need for a way to repair a fairlead on a spar-type vessel without "ballasting" the vessel. The present invention has been contemplated to meet this need.